What I had to do to make my own iPhone free ringtones was in iTunes, use "Add File to Library" (under the file menu) to add the ringtone, this will place it in its own Tones library (if you didn't already have one).
Then you can connect the iPhone, and there will be a specific sync tab for Tones where you can select what you added to the library and then sync it to your phone.
More info: http://www.iphone-mac.com/